The informal form is used among very close friends or when talking to a younger person who is very close to you in informal situations.

The formal form and the honorific form are used in formal situations. However, if I were to choose which one to use, I would choose the honorific form because the formal form is a bit casual-ish. It is safe to use the honorific form if you are unsure which one to use.

Note: The honorific form of verbs is present in Korean such as:

먹다 → 드시다 = eat
자다 → 주무시다 = sleep
죽다 → 돌아가시다 = die, pass away

먹어's honorific form is 드세요. It is usually used when talking to people above your age or in formal situations.

Rules

Informal positive (Imperative) = Present positive (spoken Korean)

해! = Do!

저리 가! = Go over there! (Leave me alone!)

이 과자 좀 먹어. = Eat some of these snacks.

물 좀 마셔. = Drink some water.

Informal negative = Take 다 off a plain form of verbs and attach 지마 to it.

가지마! = Don't go!

이 영화는 보지마! = Don't watch this movie!

이거는 먹지마. = Don't eat this.

이 책은 읽지마. = Don't read this book.

Formal positive and negative = Attach 요 to informal forms

Honorific positive =
1. Take 다 off a plain form of verbs and then attach 세요 to verbs without a final consonant.
2. Attach 으세요 to verbs with a final consonant.
3. For verbs with a final consonant of ㄹ, drop it off and attach 세요 to it.

가세요! = Please go!

이리 오세요. = Please come here.

빨리 떠나세요! = Please leave quickly!

어서 드세요. = Please eat already.

물고기 손으로 잡으세요. = Please catch the fish with your hands.

손 흔드세요! = Please wave your hands! (흔들다 = wave)

빙글빙글 도세요! = Please turn round and round! (돌다 = turn round, spin)

Honorific negative = Take 다 off a plain form of verbs and attach 지마세요.